This invention relates to an insulated wire or cable suitable for marine and sub-sea applications.
Because marine and sub-sea cables are exposed to very demanding conditions, sea water being potentially corrosive as well as electrically conductive, with sea currents giving rise to considerable mechanical stresses, they have hitherto been of relatively large diameter, but having relatively low and small temperature ranges and being physically less tough than might be desired.
Undersea cables are known which have an inner sheath of a highly insulating polymer such as polyvinyl chloride (PVC) and an outer covering of an inert polymer, for example a fluorinated polymer such as polytetrafluroethylene (PTFE).
A typical sub-sea tether or marine umbilical cable could contain a number of primary wires consisting of a conductor (typically copper or steel) surrounded by an insulating jacket (typically a thick walled cross linked polyethylene (XLPE) though un-crosslinked PE and polypropylene are sometimes used). These primary wires may then be protected by a armoured jacket consisting of metal wires (typically steel or copper wires) or aramid fibres, surrounded by an outer jacket (typically XLPE).
To provide the necessary electrical resistance and temperature rating a thick walled XLPE primary wire is traditionally used in these off-shore marine applications. The limitations of this design are that the thick walled (2.0 mm and over) primary wire results in a large diameter for the overall cable, and therefore limits the length of cable that can be stored on a single drum; thus in turn limits the length of, for example, a submarine tether cable. These wires are also limited in temperature range and physical attributes.
The present invention provides a primary wire for a marine or undersea cable having a conductive core and an insulating sheath, the sheath having an inner layer of a radiation-crosslinked polyalkene as a primary insulation, with a wall thickness of at least 0.35 mm, and an outer jacket of radiation-cross linked polyvinylidene fluoride (PVdF) having a thickness of at least 0.15 mm.
It has been found, surprisingly, that insulated wires in accordance with the invention can have the high insulation and other electrical characteristics of normal XLPE wires, while having a high temperature range, better mechanical properties such as flexibility and physical toughness and the corrosion resistance required for sub-sea, marine and offshore applications, while being substantially thinner and lighter than conventional XLPE wires. Marine cables incorporating the wires of the invention are tough and strong, abrasion resistant, resistant to chemical attack and highly flexible, with high electrical insulation and a temperature range from −55 to +150° C. This can be achieved by a synergistic combination of bespoke conductor and dual wall insulation.
Wires in accordance with the invention have particular utility as primary wires for marine or undersea cables. In some embodiments both layers are radiation-cross linked Wires of the invention can be made with a total wall thickness of around 0.8 mm, significantly thinner than conventional PE wires traditionally used in these tether and umbilical cable applications.
Additional advantages of using the wires of the invention, at least in preferred embodiments, for sub-sea cable applications include: a higher temperature range (from −55° C. to +150° C.), high electrical resistance, flexibility, corrosion resistance and physical toughness required for sub-sea, marine and offshore applications. A particular advantage of TE cables made with 44 CD wires is the low dielectric constant of the inner layer providing a lower capacitance and allowing individual wires to be bundled closer together without undesirable capacitive effects (e.g. corona effects).
The radiation crosslinking of the insulating polymers imparts increased resistance to cold flow and renders them non-melting at high temperature.
The cables of the invention may be made with metallic core conductors such as copper or with fibre optic conductors.